These are VBA coding used "AfterUpdate" event procedure.
Basically, in both cases, you hide a text box or text boxes by using the ...visible = false property of the text box. Then, depending on your approach, you display the text box.
In the first example, regardless of the "update", a text box opens. In the second, Stephen has demonstrated how you can control which text box(es) are opened depending the item selected in the combo box.
Regardless of solution...
I prefer to use
Me.Test1Txt.Visible = True
The "Me" will refer to the textbox on the current form.
There is a "gotcha" here too. You have know the name of your combo box and text box(es). To do this, open your form in "design mode", and make sure the Properties window is open - from the menu, select "View" -> "Properties". Then, select the combo or text box in question. Click on the "Other" tab. Note the value in the "Name" field -- this is the name of the text box.
I assume you are going to need more info after to consider what you want to do. So post back when you can be more specific on your requirements.
Richard